Chemical treatment of metal surface to improve paint adherence



United States Patent 3,445,298 CHEMICAL TREATMENT OF METAL SURFACE TO IMPROVE PAINT ADHERENCE Robert Rucoha, East Chicago, and William C. Slevert, Chesterton, Ind., assignors to Inland Steel Company, Chicago, Ill., a corporation of Delaware No Drawing. Filed July 21, 1966, Ser. No. 566,762

Int. Cl. C23f 7/26; C23c 3/00 U.S. Cl. 148--6.16 4 Claims The present invention relates generally to the treatment of metal surfaces and more particularly to a method of treatment of a metal surface normally susceptible to corrosion or wet storage stains, such as a galvanized surface, to providing a base having good corrosion resistance and improved paint adherent properties.

One of the compositions which is most easily applied and most effective for preventing corrosion or wet storage stains on galvanized steel or a similar metal surface which is subject to white rust is described in U.S. patent application Ser. No. 424,209, filed Jan. 8, 1965, by Uno T. Hill, now Patent No. 3,284,319, and which comprises a solution or uniform dispersion in an aqueous media or in an organic solvent solution of a trialkyl phosphate having from 3 to 8 carbon atoms per alkyl group, including straight or branched chain alkyl groups, such as tributyl phosphate and tri(2-ethyl hexyl) phosphate, and a water soluble chromate compound, such as chromic anhydride. When the foregoing composition is applied to a galvanized metal surface, a uniform film of chromium in combination with phosphate is formed on the galvanized surface and protects the galvanized surfaces against corrosion. However, in treating galvanized sheet material or similar metal surfaces with :the organic phosphate-chromate compositions of the Hill application Ser. No. 424,209, it has been found that even though corrosion resistance is consistently improved, the treated surface frequently exhibits unsatisfactory paint adherence, particularly when the corrosion resistant surface is further treated with a conventional zinc chromate conditioning solution before applying paint thereto.

It is therefore an object of the present invention to provide an improved process for chemically treating metal surfaces subject to white rust corrosion to impart improved corrosion resistance and paint adherent properties.

It is a further object of the present invention to provide an improved process of chemically treating a thin gauge galvanized strip with an organic phosphate-chromate solution so as to improve the paint adherent properties of the strip after the treated strip is contacted with a zinc chromate conversion coating.

In order to provide the desired paint-adherent surface, it has been found necessary to treat a galvanized steel strip material with the Hill organic phosphate-chromate solution in a specially controlled manner wherein both the temperature of the galvanized strip and the pH of the treating solution are maintained within predetermined limits, since it has now been found in applying the Hill organic phosphate-chromate solution that while good corrosion resistance is obtained within the specified broad pH range and at the normal ambient temperature of the strip a satisfactory paint-adherent surface cannot be obtained without maintaining the temperature of the galvanized strip and the pH of the treating solution within relatively narrow limits. Thus, when the Hill organic phosphatechromate solution is applied for the purpose of providing a satisfactory corrosion resistant surface, the pH can be maintained as high as at about pH 6. However, when a galvanized strip is treated in the foregoing manner with the Hill organic phosphate-chromate solution, and subsequently treated with a zinc chromate conversion coating prior to painting, the treated surface exhibited poor paint adherent properties. It has now been discovered that by preheating the galvanized strip to a temperature between F. and 200 F. before the strip is treated with the organic phosphate-chromate solution and with the pH of the solution being maintained between pH 2.5 and pH 3.5, consistently good paint adhesion is obtained in addition to providing good corrosion resistance.

The treating solution is preferably maintained at a temperature range between about 100 F. and 150 F., and preferably between about 100 F. and 120 F. Bath :temperatures higher than the above range increase operating expenses by causing loss of solution due to a higher evaporation rate and tend to decompose the organic ingredients of the bath, resulting in a disagreeable odor being given off by the bath. Also, higher bath temperatures cause glazing of the squeegee rolls and reduce the effectiveness of the rolls. The temperature of the treating solution is normally maintained within the specified range by having the heated metal strip enter the treating solution bath at a temperature within the range of 100 F. and 200 F.

The trialkyl phosphates which are used in the present invention are those having between 3 to 8 carbon atoms inclusive, such as tributyl phosphate, triamyl phosphate, trioctyl phosphate, and tri(2-ethyl hexyl) phosphate, as these trialkyl phosphates are water insoluble and have significant solubility for a hexavalent chromium oxide compound, such as chromic anhydride. These trialkyl phosphates also have a marked solubilizing action on boric acid and water soluble salts thereof. Thus, chromic anhydride and boric acid have been found to be solubilized or extractable into tributyl phosphate up to about ten percent (10%) by weight, respectively, at moderately elevated temperatures F.), and at room temperature tributyl, phosphate solubilizes or absorbs about eight percent (8%) by weight chromic anhydride and only slightly smaller amounts of boric acid (about 6%) on a weight basis. With the foregoing amounts of chromic anhydride dissolved or absorbed in tributyl phosphate and which preferably also contains dissolved or absorbed boric acid and the treating composition is deposited in accordance with the present invention as a uniform film on a metal surface, a phosphate-chromium composition is deposited in effective and economical amounts for improving the corrosion resistance and imparting good paintability characteristics to the metal surface.

The very significant solubility of chromic anhydride and boric acid in a trialkyl phosphate makes economically feasible and possible the further dilution, dispersion and/ or emulsification thereof in a variety of commercially available vehicles or liquid carriers which are chemically inert toward the metal surface to be treated, including both organic and aqueous vehicles. For example, a treating composition comprising tributyl phosphate which contains chromic anhydride or tributyl phosphate which contains both chromic anhydride and boric acid can be readily dissolved without impairing the effectiveness of the composition as hereinafter described in such commercially available organic solvents as naphtha, kerosene, diesel fuel, petroleum oil, gasoline, alcohol, carbon tetrachloride, trichloroethylene, benzene, toluene, Xylene, and the like organic solvents or combinations thereof.

It is preferable to use the composition in an aqueous vehicle. And, it has been found that the tributyl phosphate containing chromic anhydride or chromic anhydride with boric acid can be readily dispersed in water by adding thereto an emulsifier agent, such as a mono-, di-, or trialkylolamine, such as monoethanolamine and triethanolamine, or any other of the commonly available wetting agents, detergents, and surface acting agents including the alkyl sulfonate and aralkyl sulfonate-type emulsifying agents and ethylene oxide condensates. For example, among the emulsifying agents which can be used in the present invention are the aralkyl sulfonates, such as dodecyl benzene and tridecyl benzene sodium sulfonates and the aryl sulfonates, such as xylene sodium sulfonate and toluene sodium sulfonate, and lauryl diethanolamide, and lauryl alcohol sulfates. Also usable are the ethylene oxide adducts to fatty and rosin acids, alkyl phenol and its derivatives, the sulfated acrylic nitrogen products and sulfated esters, such as sulfated glycerol monostearate, aromatic sulfonate-OXide condensates, dioctyl ester of sodium sulfo-succinate acid, propylene monostearate, monoand di-glycerides of fatty acids, sulfated alcohol derivatives, polyoxyalkylene esters and sulfonates. It will be evident that there are a great many ionic and non-ionic emulsifiers which can be used in the present invention, and none of the commonly available emulsifiers have interfered with the satisfactory operation of the present invention when used as described herein.

Whether the composition of the present invention is applied to a surface to be treated from an organic or an aqueous solvent vehicle, a concentration of the tributyl phosphate-chromate solution is used which is sufiicient to provide a concentration of chromium on the treated surface after drying in an amount of at least about 0.25 mg. Cr per square foot and preferably between about 0.75 and 1.0 mg. Cr per square foot. Satisfactory coating on galvanized or aluminum coated strips using the composition of this invention have been produced having as much as 4.5 mg. Cr per square foot without any evidence of yellow staining which is troublesome with other chromic acid treatments at much lower levels of chromium. No practical or economic advantage is achieved, however, by providing in excess of about 1.0 mg. Cr per square foot in a coating of a galvanized steel strip 'with the present composition. And, for example, when the treating solution contains about 1% CrO by weight Cr) and about 0.5% by weight tributyl phosphate, the resultant coating deposited on a galvanized steel strip following treatment therewith is found to contain about 1.0 mg. Cr per square foot. The resultant coating also contains a proportionate amount of phosphate, borate and emulsifying agent, if the latter are used in the treating composition. Thus, in the latter embodiment the coating contains about 0.5 mg. tributyl phosphate per square foot and about 0.25 mg. boric acid per square foot.

In general, the chromic anhydride (CrO content of the treating composition on a weight percent basis can vary between about 0.1% and by weight and the tributyl phosphate or other trialkyl phosphate between about 0.1% and 99% by Weight. And, where boric acid is also included, the formulation of the treating composition on a weight percent basis can vary between about 0.1% and 10% by weight chromic anhydride, 0.1% and 10% by weight boric acid, and 80% and 99% by weight tributyl phosphate. When the treating composition is applied as an organic solvent solution the tributyl phosphate can vary between about 0.1% and 30% by Weight, the chromic anhydride between about 0.1% and 30% by weight, and the boric acid between about 0.1% and 5% by weight, with the remainder being a suitable organic solvent.

In the preferred embodiment of the present invention wherein the treating composition is applied as an aqueous solution or emulsion the ingredients thereof can be varied as follows:

Range Specific example (percent by wt.)-- (percent by wt.) 0.1-10 chromicanhydride 1.0 0.130 tributyl phosphate 1.0 01-30 emulsifying agent 0.6 O.l5 boric acid 0.4

Remainder water.

Whereas the direct combination of anhydrous chromic acid- (chromic anhydride) and tributyl phosphate at room 4 temperature results in a combustible mixture, the presence of even a relatively small amount of water with the chromic anhydride enables the chromic acid solution to dissolve smoothly and completely in the tributyl phosphate without oxidation to form a homogeneous stable chelate or complex. It is advisable to employ a chromic acid solution having a minimum concentration of 50% by weight water to prevent an exothermic oxidation reaction, and preferably a somewhat more diluted chromic acid solution (e.g. 20% by weight CrO is used when combining the chromic anhydride with the tributyl phosphate to form the homogeneous active complex. It is also possible for the tributyl phosphate which has solubilized or complexed therewith an aqueous chromic anhydride solution to solubilize or absorb therewith an appreciable amount of another metal surface protective or treating agent, such as boric acid or the like. After the homogeneous composition is formed (with or without another treating agent solubilized or absorbed thereby), the homogeneous composition can be diluted with an organic solvent in all proportions or dispersed in water with any conventional emulsifying agent, .such as an aralkyl sulfonate, or a lower alkylolamine, or by mechanical emulsifying procedures.

After application of the treating composition to the surface of an article, such as a continuous galvanized steel strip, by dipping or spraying or by any other conventional means, it is generally preferable to contact the metal surface with a wiper or squeegee or the like to ensure an even distribution of the solution on the surface of the strip and to remove any excess of the composition. Particularly good results have been obtained by employing wiper rolls fabricated from polyurethane polymer. A greater coelficient of friction between the galvanized strip and the roll is realized with polyurethane wiper rolls than between wiper rolls fabricated from rubber or neoprene polymer; thus overcoming a tendency of the wiper rolls to slip over the surface without turning. And, by coupling a set of polyurethane rolls through a belt or a chain drive to a second set of neoprene or rubber rolls a very uniform film of the treating solution is deposited upon the strip. This coupling of the rolls has also greatly reduced the need for frequent wiper roll changes. After wiping the sheet, the moisture remaining can be removed rapidly by blowing warm air over the surface or by simply allowing the coating to air dry during passage of the strip to a subsequent station.

The organic phosphate-chromate treating solution used in the present invention can be applied in-line to a continually moving strip of a protective metal or a strip having thereon a coating of a protective metal which is subject to white rust corrosion, such as zinc, aluminum, magnesium and alloys thereof, in any desired manner, including the conventional procedures of immersing the strip for a period of a few seconds (i.e. 25 seconds) in a liquid treating bath, spraying the solution onto the strip, or contacting the surface with a suitable wiper to provide a uniform wet coating. If desired, the amount of coating can be controlled by passing the wet surface under a wiper roll after application of the treating solution.

While various zinc-chromate conversion coatings are known for treating galvanized surfaces which have been treated with phosphate and chromate to improve the corrosion resistance, the preferred zinc-chromate conversion coating treatment used is that disclosed in US. Patent No. 3,130,085 to Otto. The process of the present invention, however, is not restricted to use of the Otto conversion coating composition.

The following specific example illustrates the improved process of the present invention.

EXAMPLE 1 An aqueous tributyl phosphate, chromic acid and bon'c acid treating composition was prepared as herein described and with the following composition:

Percent Tributyl phosphate, 5 gal. (by wt.) 1.05 Chromic anhydride, 36.36 lbs 0.94 Boric acid, 13.64 lbs 0.35 Monoethanolamine, 2.73 gal 0.60

Water, dil. to 450 gal Balance Twenty (20) gallons of an aqueous chromic acid solution formed by carefully dissolving 36.36 pounds of chromic anhydride therein are mixed with 5 gallons of tributyl phosphate. The chromic compound is extracted into the organic phase by the tributyl phosphate in the presence of the water without oxidation of the tributyl phosphate and reduction of the chromic compound. To the foregoing is added gallons of an aqueous solution containing 13.64 pounds of boric acid and 2.73 gallons of monoethanolamine. Water is then added to bring the volume of the composition to about 50 gallons. The foregoing ingredients are stirred and complete solubilizing or emulsification is readily effected, whereupon the composition is transferred to a treating tank containing 400 gallons of water and equipped with a recirculating pump.

The treating composition is applied by immersing in the treating tank for from 2 to 5 seconds a strip of galvanized steel which has a thickness of between about 0.018 and 0.030 inch continuously moving at a rate of about 125-150 feet per minute and which is heated from a normal temperature of about 50 F. to a temperature of between 100 F. and 200 F., and preferably to 150 F It is preferable to control the temperature of the strip within the above range by heating or cooling, as required. The galvanized strip can be heated to the above temperature range, if required, by open flame gas burner disposed on opposite sides of the strip immediately prior to contacting the strip with the treating solution. The treating solution is maintained at a temperature between 100 F. and 120 F. The strip on emerging from the treating tank is passed through a pair of oppositely disposed rubber wiper rolls of preferably about 40 durometer hardness to evenly distribute the treating composition and to remove any excess before the strip leaves the treating tank. The pressure of the wiper rolls is maintained so as to provide the desired weight of chromium in the residual coating on the treated surface (i.e. 0.75 to 1.0 mg. Cr per square foot dry coating). The coating is allowed to air dry or, if desired, warm air is passed over the surface of the strip after it passes the wiper rolls.

The pH of the treating composition is conveniently maintained between the specified limits of pH 2.5 and 3.5 by testing the preparation with a pH meter. When the pH is above 3.5 the pH is too high, and one quart at a time of aqueous chromic acid solution is added until the pH reaches the desired level. A period of about minutes should elapse between additions of the chromic acid solution. If the pH of the solution is too low (i.e. below pH 2.5), a quart of aqueous monoethanolamine is added in the above manner until the pH again is between about pH 2.5 and 3.5.

Thereafter, the strip is treated at room temperature with a chromate conversion coating solution of the type claimed in the U.S. Patent No. 3,130,085 to Otto by means of sprays which flood the surface of the moving strip with the solution. A squeege roll of the foregoing type is used to remove excess solution. The solution is allowed to remain on the metal surface between 5 and 15 seconds, with 10 seconds being preferred. The solution is then rinsed with cold water followed by a warm water rinse (120-130 F.) to assist in speeding the drying of the coating solution. A chromate conversion coating of about 3-10 mg. Cr per square foot total chromium is deposited on the metal surface. The combined coatings resulting from the organic triloweralkyl phosphatechromate treatment and the chromate conversion coating of the foregoing type or equivalent chromate conversion coating treatments provide a metal surface having superior corrosion resistance and optimum paint adherent properties.

The combined chromate coatings as above described provide an excellent base for epoxy, alkyd melamine, and acrylic base systems. For example, excellent results are obtained when using an epoxy primer paint coating ap plied to a dry thickness of about 0.1-0.2 mils and cured to ametal base temperature of about -350375 F., followed with a finish coat of an alkyd melamine applied to a dry thickness of 0.8-1.0 mils and cured to a metal base temperature of 400 440 F.

We claim:

1. A method of continuous in-line treatment of a surface of an endless strip of metal having a surface of a metal susceptible to damage by white rust corrosion which comprises; contacting a surface of said strip while preheated to a temperature between about F. and 200 F. and moving continuously past a treating zone with a liquid treating composition having as an essential active ingredient between about 0.1% and about 30% by weight trialkyl phosphate having from 3 to 8 carbon atoms per alkyl group in combination with between about 0.1% and 10% by weight of a water soluble chromate and having at least a portion of said chromate solubiliz ed by said trialkyl phosphate to form a solution of said chromate in said trialkyl phosphate, said liquid treating composition having a temperature between about 100 F. and 150 F. and having a pH between about pH 2.5 and 3.5, continuously forming on the surface of said metal strip as said strip moves past said treating zone a uniform wet film containing said solution of said chromate in said phosphate, and continuously drying said wet film to form a residual, dry film containing chromium in an amount between about 0.25 and about 4.5 mg. per square foot of metal surface; whereby said strip when treated with a chromate conversion coating provides an improved paint adherent surface.

2. A method as in claim 1, wherein said composition is a fluid aqueous dispersion comprising between about 0.1% and about 30% by weight tributyl phosphate and between about 0.1% and about 10% by weight chromic anhydride with at least a portion of said chromic anhydride being solubilizedby said tributyl phosphate to form a solution containing chromate dissolved in said tributyl phosphate and said aqueous dispersion containing between about 0.1% and about 30% by weight of an emulsifying agent.

3. A method as in claim 1, wherein said composition comprises an aqueous dispersion containing on a weight basis about 1% tributyl phosphate, about 1% chromic anhydride, about 0.4% boric acid, about 0.6% monoethanolamine, and the balance of said composition being essentially water.

4. A method as in claim 3, wherein said strip is preheated to a temperature of about 150 F. and said liquid treating composition has a temperature between about 100 F. and F.

References Cited UNITED STATES PATENTS 2,080,299 5/1937 Benning et a1. 148--6.15 X 2,224,695 12/1940 Prutton 1486.15 2,840,498 6/1958 Logue et al 1486.15 3,160,532 12/1964 Neish 1486.16 3,284,319 11/1966 Hill 1486.l6 X

RALPH S. KENDALL, Primary Examiner.

U.S. Cl. X.R. 

1. A METHOD OF CONTINUOUS IN-LINE TREATMENT OF A SURFACE OF AN ENDLESS STRIP OF METAL HAVING A SURFACE OF A METAL SUSCEPTIBLE TO DAMAGE BY WHITE RUST CORROSION WHICH COMPRISES; CONTACING A SURFACE OF SAID STRIP WHILE PREHEATED TO A TEMPERATURE BETWEEN ABOUT 100*F. AND 200*F. AND MOVING CONTINUOUSLY PAST A TREATING ZONE WITH A LIQUID TREATING COMPOSITION HAVING AN ESSENTIAL ACTIVE INGREDIENT BETWEEN ABOUT 091% AND ABOUT 30% BY WEIGHT TRIALKYL PHOSPHATE HAVING FROM 3 TO 8 CARBON ATOMS PER ALKYL GROUP IN COMBINATION WITH BETWEEN ABOUT 0.1% AND 10% BY WEIGHT OF A WATER SOLUBLE CHROMATE AND HAVING AT LEAST A PORTION OF SAID CHROMATE SOLUBILIZED BY SAID TRIALKYL PHOSPHATE TO FORM A SOLUTION OF SAID CHROMATE IN SAID TRIALKYL PHOSPHATE, SAID LIQUID TREATING COMPOSITION HAVING A TEMPERATURE BETWEEN ABOUT 100*F. AND 150*F. AND HAVING A PH BETWEEN ABOUT PH 2.5 AND 3.5 CONTINUOUSLY FORMING ON THE SURFACE OF SAID METAL STRIP AS SAID STRIP MOVES PAST SAID TREATING ZONE A UNIFORM WET FILM CONTAINING SAID SOLUTION OF SAID CHROMATE IN SAID PHOSPHATE, AND CONTINUOUSLY DRYING SAID WET FILM TO FORM A RESIDUAL DRY FILM CONTAINING CHROMIUM IN AN AMOUNT BETWEEN ABOUT 0.25 AND ABOUT 4.5 MG. PER SQUARE FOOT OF METAL SURFACE; WHEREBY SAID STRIP WHEN TREATED WITH A CHROMATE CONVERSION COATING PROVIDES AN IMPROVED PAINT ADHERENT SURFACE. 